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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/97445] Some fonctions marked static inline in Linux kernel
 are not inlined
Date: Tue, 20 Oct 2020 14:18:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97445

--- Comment #41 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is documented to be undefined:
 -- Built-in Function: int __builtin_clz (unsigned int x)
     Returns the number of leading 0-bits in X, starting at the most
     significant bit position.  If X is 0, the result is undefined.
Especially GCC 11 (but e.g. clang too) will e.g. during value range propaga=
tion
assume that e.g. the builtin return value will be only 0 to 31, not to 32, =
etc.
The portable way how to write this is x ? __builtin_clz (x) :
whatever_value_you_want_for_clz_0;
and the compiler should recognize that and if the instruction is well defin=
ed
for 0 and matches your choice, use optimal sequence.=